1. Field of the Invention
The present invention relates to a positive-type image-forming material which enables image recording by exposure to an infrared laser and increases solubility of a recording layer of an exposed area, and a planographic printing plate precursor using the same. More specifically, it relates to an image-forming material which enables writing by heating through exposure to a near infrared region of an infrared laser or the like and which is especially appropriate for a planographic printing plate precursor for so-called direct plate-making in which plate-making can directly be conducted from digital signals of a computer or the like.
2. Description of the Related Art
In recent years, with the development of a solid state laser and a semiconductor laser having an emission region from a near infrared region to an infrared region, the use of these infrared lasers has attracted much interest as a system of direct plate-making from digital data of a computer.
A positive-type planographic printing plate precursor for an infrared laser which is used for direct plate-making is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 285,275/1995. This invention is an image recording material obtained by adding a substance which absorbs light to generate heat and a positive-type photosensitive compound such as a quinonediazide compound to an aqueous alkaline solution-soluble resin, in which an image is formed such that the positive-type photosensitive compound acts as a dissolution inhibitor to substantially decrease solubility of the aqueous alkaline solution-soluble resin in an image area whereas it does not exhibit dissolution inhibitory properties through heat decomposition and is removed by development in a non-image area. Since quinonediazide compounds are photosensitive materials, an image recording material containing the same is problematic in that, for example, discoloration tends to occur when handled under a white lamp. Meanwhile, without the addition of quinonediazide compounds, a positive image can be obtained. However, in an image recording material from which the quinonediazide compound is excluded, there arises a defect that stability of sensitivity to varying concentrations of a developing solution, namely, a development latitude, is worsened.
Generally, in a positive-type planographic printing plate material capable of recording through heating with an infrared laser, a difference between a dissolution resistance to a developing solution of an unexposed area (image area) and a solubility of an exposed area (non-image area) under various use conditions is not satisfactory, and there has been a problem that excess development or insufficient development tends to occur owing to change in use conditions. Further, there have been problems that even when the surface condition is affected by a minute change by, for example, a touch to the surface in handling, an unexposed area (image area) is caused to dissolve during development, leading to formation of a defect, and further causes a shortened press life or poor ink-receptivity.
Such problems result from a substantial difference in plate-making mechanism between a positive-type planographic printing plate material used for an infrared laser exposure and a positive-type planographic printing plate material used for plate-making through UV exposure. That is, in a positive-type planographic printing plate material used for plate-making through UV exposure, an aqueous alkaline solution-soluble binder resin and onium salts or quinonediazide compounds are included as essential components, and the onium salts or quinonediazide compounds not only act as a dissolution inhibitor through interaction with a binder resin in an unexposed area (image area) but also serve as a dissolution accelerator by generating an acid through decomposition by light in an exposed area (non-image area), thus playing two roles.
On the other hand, IR dyes and other dyes included in a positive-type planographic printing plate material used with an infrared laser merely act as a dissolution inhibitor for an unexposed area (image area), and do not act to accelerate dissolution in an exposed area (non-image area). Accordingly, in order to exhibit a difference in solubility between an unexposed area and an exposed area, a positive-type planographic printing plate material used for an infrared laser is required to employ a resin having a high solubility in an alkaline developing solution as a binder resin from the start, resulting in an unstable condition before development. As such, a positive-type planographic printing plate material has a limitation in storage conditions before recording and has a problem in improving storability with the passing of time.
With respect to improvement of a development latitude, for example, in order to increase dissolution resistance to a developing solution at an unexposed area (image area) without impairing developability of an exposed area (non-image area), JP-A No. 1-288,093 proposes a method which uses a copolymer composed of an addition-polymerizable fluoro-containing monomer having in a side chain a fluoroaliphatic group in which a hydrogen atom bonded to a carbon atom has been substituted with a fluorine atom, and EP 950517 proposes a method which uses a siloxane-based surfactant. Although these methods may contribute to improve resistance to development at an image area in the recording layer to some extent, a difference in solubility between an unexposed area and an exposed area is not large enough to form a sharp and good image considering fluctuation of activity of a developing solution.
It is an object of the present invention to provide a positive-type image-forming material which is excellent in latitude during image-forming through development and in scratch resistance and good in storability, and also to provide a positive-type planographic printing plate precursor which has a recording layer exhibiting such excellent properties and can be used for direct plate-making using an infrared laser.
The present inventors conducted extensive researches to improve development latitude, scratch resistance and storability, and found that the foregoing objects can be attained by addition of a phenol compound having a specific structure. This finding has led to completion of the present invention.
That is, the present invention provides the following.
 less than 1 greater than  A heat mode-compatible positive-type image-forming material including: (a) a water-insoluble, aqueous alkaline solution-soluble polymer compound (hereinafter occasionally referred to as an alkali-soluble resin), (b) a light-heat converting agent and (c) a phenol including a partial structure represented by the following formula (I) (hereinafter occasionally referred to as a specific phenol compound), the positive-type image-forming material exhibiting an increase in solubility in an aqueous alkaline solution when the positive-type image-forming material is heated: 
wherein: X represents a monovalent terminal group having 2 or more carbon atoms or a linking group represented by xe2x80x94CY1Y2xe2x80x94 or xe2x80x94CHY1xe2x80x94 in which Y1 and Y2 each represent monovalent terminal groups having 1 or more carbon atoms; W represents a monovalent terminal group; and n represents an integer of 1 to 4.
 less than 2 greater than  A planographic printing plate precursor in which a recording layer made of a positive-type image-forming material that includes (a) a water-insoluble, aqueous alkaline solution-soluble polymer compound, (b) a light-heat converting agent and (c) a phenol including a partial structure represented by formula (I) is formed on a substrate, the positive-type image-forming material exhibiting an increase in solubility in an aqueous alkaline solution when the positive-type image-forming material is heated.
Although the functional mechanism of the present invention is not elucidated, the compound represented by formula (I) carries a bulky substituent having a relatively high molecular weight at the o-position, and in such compounds having a bulky substituent at the o-position of the phenolic hydroxyl group, the hydroxyl group is sterically masked. It is therefore presumed that addition of the compound (c) enhances a hydrophobic nature of the phenol compound and allows steric masking of the phenolic hydroxyl group, and when an interaction occurs with the alkali-soluble resin (a) used in combination, the compound (c) also masks a hydroxyl group present in the alkali-soluble resin, whereby alkali permeation is inhibited in an unexposed area and thus scratch resistance of a photosensitive material is improved.
Since the compound (c), although having a bulky group, is a low-molecular compound, inhibition can readily be released through exposure, to allow an increased solubility at a heated portion to thus enhance development latitude. Further, being a low-molecular compound, the compound (c) is considered to be able to exert an improved storability by creating a firm interaction network with the alkali-soluble resin (a), thereby suppressing a change in interaction with the passing of time.
Incidentally, xe2x80x9cheat mode-compatiblexe2x80x9d in the present invention means that recording by heat-mode exposure is possible. The definition of the heat-mode exposure in the present invention is described in detail. As stated in Hans-Joachim Timpe, IS and Ts NIP 15:1999 International Conference on Digital Printing Technologies, p. 209, there are known two modes in a process starting from photo-excitation of a light absorbing material (for example, dyes) effected in a photosensitive material to a chemical or physical change, in the case where photo-excitation is caused in the material resulting in a chemical or physical change to finally form an image. One is a so-called photon mode in which a photo-excited light absorbing material is deactivated by creating some photochemical interaction (for example, energy transfer or electron transfer) with other reactants in a photosensitive material so that the activated reactants induce a chemical or physical change required for the image-forming. Another is a so-called heat mode in which a photo-excited light absorbing material is deactivated by generating heat and, by utilizing the generated heat, reactants induce a chemical or physical change required for the image-forming. There are known additional specific modes, such as abrasion in which a substance is exploded and scattered by an action of energy of light gathered locally or multiphoton absorption in which one molecule absorbs a large number of photons at a time. However, description of these specific modes is omitted herein.
The exposure processes using the foregoing modes are called photon-mode exposure and heat-mode exposure, respectively. A technical difference between the photon-mode exposure and the heat-mode exposure depends on whether or not an energy amount of some photons for exposure can be added to reach an energy amount of an intended reaction. For example, suppose that a reaction is conducted using n photons. Since the photon-mode exposure utilizes a photochemical interaction, accumulative use of energies of individual photons is impossible according to the law of conservation of energy and momentum of quantum. That is, in order to induce any chemical reaction, a relation defined by xe2x80x9cenergy amount of 1 photonxe2x89xa7energy amount of reactionxe2x80x9d is required. Meanwhile, in the heat-mode exposure, heat is generated after photo-excitation, and light energy is converted into heat and then used. Accordingly, accumulation of the energy amounts is possible. Thus, a relation defined by xe2x80x9cenergy amount of n photonsxe2x89xa7energy amount of reactionxe2x80x9d is sufficient, with a proviso that there is a limitation to the addition of the energy amount due to the occurrence of heat diffusion. That is, when the subsequent photo-excitation-deactivating process occurs to generate heat before heat is lost from an exposed area (reaction point) through heat diffusion, heat is surely accumulated and added, which leads to an increase in temperature of the affected portion. However, when the subsequent heat generation is delayed, heat is lost, and not accumulated. That is, in heat-mode exposures with the same total exposure energy amount, the results produced are different between the application of light having a large amount of energy for a short period of time and the application of light having a small amount of energy for a long period of time. The application of light for a short period of time is advantageous for accumulation of heat.
Of course, in the photon-mode exposure, a similar phenomenon sometimes occurs due to diffusion of materials effected in the subsequent reaction. However, such a phenomenon basically does not occur.
That is, in view of characteristics of a photosensitive material, in the photon-mode, an inherent sensitivity (energy amount for a reaction required for image-forming) is fixed to a specified value relative to an exposure power density (w/cm2) (=energy density per unit time). However, in the heat-mode, an inherent sensitivity of a photosensitive material is increased relative to an exposure power density. Accordingly, when an exposure time in which a productivity required for an image recording material can actually be maintained is fixed, high sensitization of approximately 0.1 mJ/cm2 can be attained in the photon-mode exposure when comparing the respective modes with one another. However, no matter how small the amount of exposure is, a reaction may occur. Therefore, a problem of fogging is likely to arise due to low exposure in an unexposed area. On the other hand, in the heat-mode exposure, a reaction does not take place unless an exposure amount is more than a specified value. From the relation to heat stability of a photosensitive material, approximately 50 mJ/cm2 is usually required, but a problem of fogging due to low exposure is avoided.
In fact, in the heat-mode exposure, an exposure power density in a plate surface of a photosensitive material has to be 5,000 w/cm2 or more, preferably 10,000 w/cm2 or more. However, it is not preferred to use a high power density laser of more than 5.0xc3x97105 w/cm2 because of a problem that abrasion will occur to taint a light source, which has not been stated in detail herein.